Fluorescent inks and toners are among the most widely used security printing features. A printed document is usually authenticated by detecting the light emitted by the fluorescent component when subjected to black light. The light emitting property cannot be reproduced in a second generation copy.
Fluorescent dyes used in fluorescent inks and toners can lose fluorescence in the print-head when the ink is heated to a temperature greater than 120° C. to melt during normal operation. To overcome this problem, the security printing industry uses hard, robust pigments containing the dye of interest. Pigments are preferred over fluorescent dyes because of their improved chemical and light fastness as well as improved thermal stability. Pigments are also preferred by the industry because there is limited or no migration or bleeding of the colorant as generally observed with dye compounds.
Most commercially available fluorescent pigments are made by grinding a bulk polymer matrix containing fluorescent materials. This process does not result in fluorescent particles of a size smaller than 1-2 microns, and typically the size of these particles is about 4-5 microns. According to this process, fluorescent dyes are incorporated into hard, crosslinked particles, thereby limiting the mobility of the fluorescent dye. Once the fluorescent dye is isolated from interaction with other materials present in the ink and, chemical degradation by the environment is diminished. These hard particles are dispersed in the marking material, typically liquid inks.
Inks based on fluorescent pigments are currently used in rotogravure, flexographic, silk-screening and off-set printing systems. However, given their large size, inks based on these pigments cannot be used with inkjet, solid ink or UV curable inks, because they physically clog the inkjet nozzles.
Thus, there is a need in the art for fluorescent compositions, including fluorescent compositions that can be used in/with inkjet inks, solid ink, UV curable inks and EA (Emulsion Aggregation) toners and that have suitable thermal degradation properties. There is a further need for fluorescent compositions of such small size that can be used in or with inkjet inks, solid inks, UV curable inks and EA toners and be such that they are compatible with organic based marking materials.
The present disclosure addresses these needs by providing solid inks, particularly phase change ink compositions containing at least one nanoscale fluorescent pigment particle and/or at least one fluorescent organic nanoparticle, and the use of such inks in methods for forming images, particularly their use in inkjet printing.
Ink jetting devices suitable for use with the solid (or phase chance) inks of the present disclosure are known in the art, and thus extensive description of such devices is not required herein. As described in U.S. Pat. No. 6,547,380, incorporated herein by reference in its entirety, inkjet printing systems generally are of two types: continuous stream and drop-on-demand.
In continuous stream inkjet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field that adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium.
In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
There are at least three types of drop-on-demand inkjet systems. One type of drop-on-demand system is a piezoelectric device that has as its major components an ink-filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. Another type of drop-on-demand system is known as acoustic ink printing. An acoustic beam exerts a radiation pressure against objects upon which it impinges. When an acoustic beam impinges on a free surface (that is, liquid/air interface) of a pool of liquid from beneath, the radiation pressure that it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. Still another type of drop-on-demand system is known as thermal inkjet, or bubble jet, which produces high velocity droplets. The major components of this type of drop-on-demand system are an ink-filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink vehicle (usually water) in the immediate vicinity to vaporize almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands.
In general, phase change inks are in the solid phase at, for example, ambient or room temperature, such as about 20° C. to about 27° C., but exist in the liquid phase at the elevated operating temperature of an inkjet printing device. At the jet operating temperature, the ink is molten and droplets of liquid ink are ejected from the printing device. When the ink droplets contact the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, they quickly solidify to form a predetermined pattern of solidified ink drops.
In a typical design of a piezoelectric inkjet device utilizing phase change inks printing directly on a substrate or on an intermediate transfer member, such as the one described in U.S. Pat. No. 5,372,852, incorporated herein by reference, the image is applied by jetting appropriately colored inks during four to eighteen rotations (incremental movements) of a substrate (an image receiving member or intermediate transfer member) with respect to the ink jetting head; that is, there is a small translation of the printhead with respect to the substrate in between each rotation. This approach simplifies the printhead design, and the small movements ensure good droplet registration.
Hot melt inks typically used with inkjet printers have a wax based ink vehicle, for example, a crystalline wax. Such solid inkjet inks provide vivid color images. In typical systems, these crystalline wax inks partially cool on an intermediate transfer member and are then pressed into the image receiving medium such as paper, spreading the image droplet, providing a richer color and lower pile height. The low flow of the solid ink also prevents show through on the paper.
In these systems, the crystalline wax inks are jetted onto a transfer member, for example, an aluminum drum, at temperatures of approximately 100-140° C. The wax based inks are heated to temperatures sufficient to decrease their viscosity for efficient and proper jetting onto the transfer member. The transfer member is at approximately 60° C., so that the wax will cool sufficiently to solidify or crystallize. As the transfer member rolls over the recording medium, for example, paper, the image comprised of wax based ink is pressed into the paper.
However, the use of crystalline waxes places limitations on the printed image. Conventional crystalline waxes are apolar aliphatic molecules bound together by weak van der Waals forces. These waxes have little natural affinity for the more polar paper substrate and, due to the nature of their own intermolecular bonding, are vulnerable to mechanical damage.
U.S. Pat. No. 6,906,118, incorporated herein by reference in its entirety, discloses phase change ink compositions wherein at a first temperature, hydrogen bonds of sufficient strength exist between the ink vehicle molecules so that the ink vehicle forms hydrogen bonded dimers, oligomers, or polymers, and wherein at a second temperature higher than the first temperature, the hydrogen bonds between the ink vehicle molecules are sufficiently broken that fewer hydrogen-bonded dimers, oligomers, or polymers are present in the ink at the first temperature so that the viscosity of the ink at the second temperature is lower than the viscosity of the ink at the first temperature.
While known compositions and processes are suitable for their intended purposes, a need remains for improvements in ink jetable phase change inks, for example, for solid (or phase change) inks that exhibit fluorescence with suitable degradation properties.
Typically, dyes have been the colorants of choice for inkjet printing inks because they are readily soluble colorants and, more importantly, do not hinder the reliable jetting of the ink. Dyes have also offered superior and brilliant color quality with an expansive color gamut for inks, when compared with conventional pigments. However, because dyes are molecularly dissolved in the ink vehicle, they are often susceptible to unwanted interactions that lead to poor ink and print quality performance. Examples include photooxidation from light leading to poor lightfastness, dye diffusion from the ink into paper or other substrates leading to poor image quality and print showthrough, and the ability for the dye to leach into another solvent that makes contact with the image leading to poor water and/or solvent fastness.
Key issues precluding pigment particle use for inkjet inks are their large particle sizes and wide particle size distribution, the combination of which can pose critical problems with reliable jetting of the ink (i.e. inkjet nozzles are easily blocked). Pigments are obtained as large aggregates of crystals and with wide distribution of aggregate sizes. The color characteristics of the pigment aggregate can vary widely depending on the aggregate size and crystal morphology. Thus, an ideal colorant that is widely applicable in, for example, inks and toners, is one that possesses the best properties of both dyes and pigments, namely: 1) superior coloristic properties (large color gamut, brilliance, hues, vivid color); 2) color stability and durability (thermal, light, chemical and air-stable colorants); 3) minimal or no colorant migration; 4) processable colorants (easy to disperse and stabilize in a matrix); and 5) inexpensive material cost.
The following documents provide further background information:
U.S. Pat. No. 6,902,613 discloses a mixture of an organic nanosize pigment comprising of from 50 to 99% by weight of the nanosize pigment and 1 to 50% by weight based of a low molecular weight naphthalene sulfonic acid formaldehyde polymer and its use as a particle growth and crystal phase director for the preparation of a direct pigmentary organic pigment or in pigment finishing.
WO 2004/048482 discloses a mixture of an organic nanosize pigment comprising of from 50 to 99% by weight of the nanosize pigment and 1 to 50% by weight based of a low molecular weight polysulfonated hydrocarbon, in particular naphthalene mono- or disulfonic acid formaldehyde polymer, and its use as a particle growth and crystal phase director for the preparation of a direct pigmentary organic pigment or in pigment finishing.
U.S. Patent Application Publication No. 2006/0063873 discloses a process for preparing nano water paint comprising the steps of: A. modifying the chemical property on the surface of nano particles by hydroxylation for forming hydroxyl groups at high density on the surface of the nano particles; B. forming self-assembly monolayers of low surface energy compounds on the nano particles by substituting the self-assembly monolayers for the hydroxyl groups on the nano particles for disintegrating the clusters of nano particles and for forming the self-assembly monolayers homogeneously on the surface of the nano particles; and C. blending or mixing the nano particles having self-assembly monolayers formed thereon with organic paint to form nano water paint.
U.S. Pat. No. 7,160,380 describes a method of producing a fine particle of an organic pigment, containing the steps of: flowing a solution of an organic pigment dissolved in an alkaline or acidic aqueous medium, through a channel which provides a laminar flow; and changing a pH of the solution in the course of the laminar flow.
Thus, there is a need, which is addressed by embodiments of the present disclosure, for smaller fluorescent particles that minimize or avoid the problems associated with conventional larger-sized pigment particles. There further remains a need for processes for making and using such improved nanoscale fluorescent particles as colorant materials. Such nanoscale fluorescent particles are useful in, for example, paints, coatings and inks (e.g., inkjet printing inks) and other compositions where pigments can be used such as plastics, optoelectronic imaging components, photographic components, and cosmetics among others.